1. Field of the Invention
The present invention relates to a semiconductor device for surface-shape recognition, more particularly relates to an electrostatic capacity type semiconductor device for surface-shape recognition for sensing fine topology of human fingerprints etc.
2. Description of the Related Art
Due to the growth of the information society, interest has risen in security in modern society. For example, in an information society, personal authentication has become an important key in constructing electronic cashing and other systems. Further, much research activity is going on regarding authentication as a defensive measure against theft and illicit use of credit cards.
Accordingly, much technology has been disclosed regarding surface shape recognition as represented by fingerprint sensors.
Here, the methods of detection in fingerprint and other shape recognition includes the optical detection method and the electrostatic capacity detection method.
The electrostatic capacity detection method is a method for detecting the value of the electrostatic capacity (hereinafter also simply referred to as a capacity value) between an electrode of a shape recognition sensor and for example a finger. The electrostatic capacity type is advantageous for mounting in a portable terminal etc. since it enables easy reduction of the size of the device, so there is active work being conducted on development of electrostatic capacity type sensors.
Here, an explanation will be made of a semiconductor device for surface-shape recognition according to the related art. Specifically, an explanation will be made of one for sensing the fine topology of human fingerprints etc.
FIG. 1 is a sectional view of an electrostatic capacity type semiconductor device for surface-shape recognition.
A gate electrode 30 forming a word line is formed above a channel formation region of a semiconductor substrate 10 via a not illustrated gate insulating film. Further, source and drain diffusion layers 11 are formed in the semiconductor substrate 10 at the two side portions of the gate electrode 30. Thus, a transistor Tr is formed. One of the source and drain diffusion layers 11 of the transistor Tr is connected to a not illustrated bit lines.
An inter-layer insulating film 20 made of for example silicon oxide is formed covering the transistor Tr. Sensor pad electrodes 31 each formed by a laminate of a barrier metal layer made of for example Ti and an aluminum layer etc. are formed at an upper layer thereof while arranged in a matrix. A sensor pad electrode 31 is formed connected to the other source or drain diffusion layer 11 of the transistor Tr formed in a lower layer thereof by a not illustrated contact etc.
A first protective film 21 of an insulator made of for example a silicon nitride is formed over the entire surface while covering the sensor pad electrodes 31 and clearances between the sensor pad electrodes 31. A neutralization electrode 32a fixed to a certain potential and made of for example Ti is formed at an upper layer of the first protective film 21. A second protective film 22 of an insulator made of for example silicon nitride is formed over the entire surface while covering the first protective film 21 and the neutralization electrode 32a. Here, the surface of the second protective film 22 at the upper portion of the neutralization electrode 32a forms a convex shape M.
As described above, a semiconductor device for surface-shape recognition using a region wherein the sensor pad electrodes 31 are arranged in a matrix as a shape recognition surface is formed.
Next, an explanation will be made of the operation of a semiconductor device for surface-shape recognition.
As shown in FIG. 2A, when for example a human finger 7 touches the shape recognition surface of the semiconductor device for surface-shape recognition, capacitors are formed from the sensor pad electrodes 31, the first protective film 21 and the second protective film 22, and the finger 7. The first protective film 21 and the second protective film 22 act as part of the capacitor insulating film. In the above description, the distances d between the sensor pad electrodes 31 and the finger 7 (for example d1, d2, . . . ) fluctuate in accordance with the topology 70 of the fingerprint. Accordingly, there arises a difference in the capacitances of the capacitors formed by the sensor pad electrodes 31 arranged above the shape recognition surface in the matrix. Therefore, it has become possible to recognize the shape of a fingerprint etc. by reading and detecting charges stored in the sensor pad electrodes 31 by a semiconductor element such as a transistor formed on the substrate 10.
Here, each sensor pad electrode 31 forms a unit cell of the shape recognition surface of the semiconductor device for surface-shape recognition.
The capacitors configured by the sensor pad electrodes 31 have distances d equal to ∞ in all unit cells of the shape recognition surface of the semiconductor device for surface-shape recognition in a state where the finger 7 or the like does not touch the 10 shape recognition surface. Accordingly, the electrostatic capacity value Cs becomes equal 0 in all unit cells.
On the other hand, in a state where the finger 7 or the like touches the shape recognition surface, as shown in FIG. 2B, in an n-th unit cell, capacitors of the electrostatic capacity value CSn are formed from the sensor pad electrode 31, the first protective film 21 and the second protective film 22, and the finger 7. The electrostatic capacity value CSn is represented by:
CSn=xcex5xc2x7xcex50xc2x7S/dn
Here, S is the area contributing to the capacitor of each electrode, dn is a distance between the electrode of the n-th unit cell and the finger (for example d1, d2, . . . ), and n is the number of each unit cell (n=1, 2, 3, . . . ).
As the configuration for reading the electrostatic capacity value CSn in the unit cells, there is employed a configuration wherein the capacitors formed from the sensor pad electrode 31 of each unit cell, the first protective film 21 and the second protective film 22, and the finger 7 are connected to one source or drain diffusion layer 11 of the transistor using for example a word line WL (WL1, WL2, . . . ) as the gate electrode, the other source or drain diffusion layer 11 is connected to a bit line BL (BL1, BL2, . . . ), and further a capacitor of a electrostatic capacity value CB is connected to the bit line BL.
In the above configuration, by the touch of the finger in a state where VCC is applied to the bit line BL (VCC precharge), a potential change of the bit line BL represented by:
xcex94Vn=[CSn/(CB+CSn)]xc2x7VCC
occurs. By detecting this potential change xcex94Vn in each cell, the electrostatic capacity value CSn for every unit cell is calculated and image processing is performed to recognize the shape of the object.
Here, for example the human body etc. is generally sometimes charged. Therefore, in the conventional semiconductor device for surface-shape recognition, as shown in FIG. 2A, in order to prevent damage of the semiconductor device for surface-shape recognition due to discharge of static electricity to the shape recognition surface when the charged human puts his finger close to the shape recognition surface of the semiconductor device for surface-shape recognition, the neutralization electrode 32a fixed at for example the ground potential is provided near the surface of the shape recognition surface.
However, since the neutralization electrode 32a is formed at a predetermined position above the first protective film 21 made of for example silicon nitride, while the second protective film 22 made of for example silicon nitride is formed while covering the entire surfaces of the neutralization electrode 32a and the first protective film 21, and the shape recognition surface forms a convex shape M accordingly, in the base portion of the convex shape M, there is insufficient mechanical strength, so there was the problem that, as shown in FIG. 3, a crack C was formed from the second protective film 22 of the shape recognition surface when pressed by a finger or the like, and the semiconductor device for surface-shape recognition was damaged.
In order to solve such a problem, the mechanical strength may be raised by flattening the surface of the shape recognition surface. As one of the processes for increasing the flatness of the surface, there is chemical mechanical polishing (CMP). In CMP, however, a new CMP system becomes necessary. Further, there is also a process using etch back such as in the following explanation. Below, an explanation will be made of the process of using etch back as another process for increasing the flatness of the surface by referring to the drawings.
First, as shown in FIG. 4A, a resist film R1 of a pattern opening the convex shape M of the second protective film 22 is formed on the second protective film 22 by photolithography.
Next, as shown in FIG. 4B, a resist is coated over the entire surface while covering the convex shape M of the second protective film 22 and the resist film R1 and thereby to form a resist film R2.
Next, as shown in FIG. 5A, by for example dry etching, the resist film R1 and the resist film R2 are etched back at the entire surface to expose the convex shape M of the second protective film 22.
Next, as shown in FIG. 5B, by using etching such as reactive ion etching (RIE), the resist film R1 and the second protective film 22 are etched back at the entire surface under conditions of substantially equivalent etching rates and part of the convex shape M of the second protective film 22 is eliminated to make the step of the convex shape M small. After the etching, a convex shape m having a small step remains. Then, the remaining resist film R1 is eliminated.
Next, as shown in FIG. 6A, a third protective film 23 is formed on the entire surface while covering the second protective film 22 by for example chemical vapor deposition (CVD). At this time, the convex shape m is formed at the surface of the third protective film 23.
Next, as shown in FIG. 6B, a SOG (Spin on Glass) film 24 is formed by coating SOG over the entire surface while covering the third protective film 23.
Next, as shown in FIG. 7A, by for example RIE, the SOG film 24 is etched back at the entire surface to expose the top surface of the convex shape m of the third protective film 23. At this time, there is almost no step between the surface of the third protective film 23 and the surface of the SOG film 24, so the surface becomes flat.
Next, as shown in FIG. 7B, silicon nitride is deposited on the entire surface while covering the third protective film 23 and the SOG film 24 by for example CVD to form a fourth protective film 25.
Even by the method using etch back, flattening of the shape recognition surface can be achieved, but the number of the steps is remarkably increased by the above method.
An object of the present invention is to provide a semiconductor device for surface-shape recognition capable of improving reliability by preventing a destruction of the device due to a discharge of static electricity and an occurrence of cracks in the shape recognition surface at the time of pressing by an object such as a finger.
According to a first aspect of the present invention, there is provided a semiconductor device for surface-shape recognition comprising a first transistor formed on a substrate; a first electrode formed on the first transistor; a first protective film formed on the first electrode; and a second electrode formed on the first protective film; the second electrode being formed in a groove formed in the first protective film.
Preferably, the thickness of the second electrode is substantially the same as the depth of the groove.
Preferably, the device further comprises a second protective film formed on the second electrode. More preferably, the first protective film and the second protective film are made of different materials.
Preferably, the second electrode is fixed to a certain potential.
Preferably, the first electrode is arranged in a matrix.
Preferably, the first transistor is a field effect transistor and the source or drain of the transistor is connected to the first electrode.
According to the semiconductor device for surface-shape recognition of the present invention, the charge stored in the first electrode can be read by the transistor. For example, due to the configuration of the other source or drain region connected to the bit line, if an object such as a finger touches the second protective film or the like in the state where a predetermined voltage is applied to the bit line, the potential of the bit line changes. By detecting the potential change of the bit line, the electrostatic capacity value of each capacitor can be read. Therefore, the surface shape of the object can be recognized. At this time, since a second electrode impressed with the fixed potential is formed, even if static electricity is discharged when pressing the object, swift neutralization is carried out by the second electrode, thus electrostatic destruction can be prevented.
Further, the surface as the shape recognition surface can be flattened, the mechanical strength is improved, and the occurrence of cracks at the shape recognition surface at the time of pressing by an object can be prevented.
As described above, the improvement of the reliability has become possible.
According to a second aspect of the present invention, there is provided a semiconductor device for surface-shape recognition comprising first and second transistors formed on a substrate; first and second electrodes formed on the first and second transistors; a first protective film formed on the first and second electrodes; and a third electrode formed on the first protective film; the third electrode being formed in a groove formed in the first protective film.
Preferably, the thickness of the third electrode is substantially the same as the depth of the groove.
Preferably, the device further comprises a second protective film formed on the third electrode. More preferably, the first protective film and the second protective film are made of different materials.
Preferably, the third electrode is fixed to a certain potential.
Preferably, the first and second electrodes are field effect transistors, the source or drain of the first transistor is connected to the first electrode, and the source or drain of the second transistor is connected to the second electrode. More preferably, the terminals which are not connected to the first and second electrodes of the first and second transistors are connected to capacitors.